scripts: fix sloppy/unpredictable cmd && this || that statements

- be a bit more POSIX to avoid any potential issues when full shell stdout/err are redirected
- actual logic chains remain unchanged

scripts/update_binary.sh

@@ -14,7 +14,8 @@ setup_bb() {
 export BBBIN
 case "$1" in
   "extract"|"-x")
-    [ -z "$2" ] && BBBIN=./busybox || BBBIN="$2"
+    BBBIN=./busybox
+    [ -z "$2" ] || BBBIN="$2"
     extract_bb
     ;;
   "sh")